The present invention relates to a method for treating a nitrogen oxide- and sulphur oxide-containing waste gas, in particular improvements in the method for removing the nitrogen oxide and sulphur oxide from said waste gas which comprises introducing the waste gas after it has been mixed with gaseous ammonia into a cross current type moving bed reactor packed with carbonaceous catalyst such as activated carbon.
The method for treating a nitrogen oxide- and sulphur oxide-containing waste gas, which comprises mixing gaseous ammonia to said waste gas and allowing the mixture of waste gas and ammonia to pass through a packed bed of carbonaceous catalyst such as activated carbon or activated carbon supporting thereon a metal oxide such as vanadium pentoxide or the like, may be said advantageous in that it permits not only the simultaneous removal of nitrogen and sulphur oxides but also the regeneration use of the catalyst. In order to remove the nitrogen oxide and sulphur oxide from waste gas efficiently by virtue of this method, however, it is necessary that the reaction temperature should be at least 200.degree. C. or more, preferably in the range of about 220.degree. C. to 250.degree. C. If the reaction temperature is below this range, it will hamper the thorough removal of nitrogen oxide.
In this context, FIG. 1 and FIG. 2 are each a graph illustrating the relation between reaction temperature of the fixed bed reactor and percentage removal of NO (FIG. 1) and between reaction temperature of the fixed bed reactor and percentage removal of SO.sub.2 (FIG. 2) in case where 500 ppm of ammonia has been injected in nitrogen gas containing 750 ppm of SO.sub.2, 200 ppm of NO, 6% of O.sub.2 and 10% of H.sub.2 O, and this mixed gas has been allowed to pass through an activated carbon catalyst fixed bed reactor for 50 hours at the flow velocity of 0.8 Nm.sup.3 /hr (which corresponds to the space velocity of 800 hr.sup.-1). As is evident from FIG. 1 and FIG. 2, in the case of the percentage removal of SO.sub.2 it is maintained at the level of 90% or more at the reaction temperature ranging from 120.degree. C. to 220.degree. C. upto 30 hours' gas supply, while in the case of the percentage removal of NO it rapidly decreases as the reaction temperature lowers and drops below the level of 80% at the reaction temperature of 200.degree. C. during about 20 hours' gas supply.
Accordingly, in order to effect the simultaneous removal of nitrogen oxide and sulphur oxide from waste gas by using a carbonaceous catalyst, the reaction temperature must be held at 200.degree. C. at the lowest, but the reaction temperature of 200.degree. C. gives rise to trouble that a part of the carbonaceous catalyst has been consumed by the oxygen in waste gas as represented by the following reaction: C+O.sub.2 .fwdarw.CO.sub.2. In addition thereto, the aforesaid method is disadvantageous in that normal combustion waste gases from boilers and the like must be heated to a temperature of 200.degree. C. or more prior to treatment because those kinds of gases ordinarily are of a temperature of about 150.degree. C. at the outlet of the air heater or the like.
And, FIG. 3 is a graph illustrating the relation between SO.sub.2 concentration and percentage removal of NO in case where the mixed gas was treated under the same conditions as those of the experiments having obtained the results as shown in FIGS. 1 and 2 with the exception that the reaction temperature of the fixed bed reactor was set 150.degree. C. and SO.sub.2 concentration was varied within the range of 0-1000 ppm. It will be clearly seen from FIG. 3 that the lower the SO.sub.2 concentration in the mixed gas is, the higher the denitrification efficiency can be maintained.
In this context, it may be said that the cross current moving bed reactor in which the gas to be treated is passed through the reactor provided with the moving bed of catalyst particles adapted to move downwards in the cross current direction, namely in the transverse direction, since the inlet side of the moving bed is always in contact with the untreated gas, in principle can be treated as an assembly of fixed beds whose layer thickness is equivalent to the layer height. This leads to that the catalyst dwell time in the moving bed corresponds to the gas supply time to the fixed bed. And, the changes in removal percentage in the fixed bed with the lapse of time correspond to the changes in removal percentage extending from the top to the bottom of the moving bed.
Accordingly, it can be analogized from FIG. 1 that in case where the reaction temperature is low, the cross current moving bed can achieve a relatively high denitrification efficiency at its upper part but only a low denitrification efficiency at its lower part. Viewed in relation to the SO.sub.2 concentration, furthermore, it can be analogized from FIG. 3 that a high denitrification efficiency can be achieved at the upper part of the cross current moving bed even when the SO.sub.2 concentration is relatively high.
In other words, it may be said that when treating the nitrogen oxide- and sulphur oxide-containing waste gas by means of the cross current moving bed reactor, the nitrogen oxide and sulphur oxide contained in the gas passing through its upper part can be removed well but the gas passing through its lower part still contains considerable quantities of nitrogen oxide and sulphur oxide.